In vivo sister chromatid exchange (SCE) analyses have been extended to rat fetal as well as maternal tissues. SCE induction effects from selected mutagens/teratogens are being examined after maternal drug metabolism and transplacental passage to various fetal tissues. In some instances, in vitro methodology is also employed in order to study direct effects from the parent compound, and also from known metabolites. The system should allow for sensitive cytological assessment of comparative drug damage at various maternal-fetal interaction stages.